The invention relates to a method for evaluating electrical characteristic values of a coupled piezo-mechanical system.
Piezo-elements are being employed with increasing frequency to control mechanical movements, for instance in fuel injectors.
The coupled system formed of a piezo-element, for instance a piezo-ceramic, and a mechanically movable mechanism that is connected to it, is referred to here as a piezo-mechanical system. The piezo-mechanical system can additionally be influenced by another mode of action, for instance by means of a hydraulic system or by an electromagnetically driven motor.
As opposed to an electromagnetic drive as is primarily in use today, the force transmission from the piezo-element to the mechanical moving parts is not performed indirectly via a force field (e.g. a magnetic field), but rather directly via a mechanical coupling (hydraulic system, tappet, etc.).
Another difference is that the piezo-element, as a mechanical-electrical converter, is both part of the mechanical action loop, or subsystem, and part of the electrical action loop, or subsystem. The coupling effect between mechanical and electrical subsystems is approximately linear in a piezo-element (that is, it follows relationships which are relatively easy to describe), unlike in an electromagnetically driven actuator, for example. This means that a modification of a technical parameter (e.g. due to wear or aging) changes the mechanical stiffness of components, thereby exerting a direct influence on the counterforce experienced by the piezo-element.
It is known that the impedance characteristic of the piezo-element as a whole, and the position of impedance maxima and minima in particular, changes when a mechanical influence quantity (such as the pressure on the piezo-element) is modified.
It is the object of the present invention to use the characteristics of a piezo-element to diagnose a piezo-mechanical system.
According to the method of the present invention for diagnosing a piezo-mechanical system which contains at least one piezo-element and a mechanical system controlled by the piezo-element, an electrical piezo-signal which is tapped at the piezo-element is compared to an electrical signal that controls the piezo-element. The condition of the piezo-mechanical system is determined from a deviation of these two signals from one another.
The object is achieved by exploiting the fact that the state of the mechanical subsystem influences the electrical characteristic values of the piezo-element (henceforth, the piezo-signal) in a characteristic fashion on the basis of the mechanical-electrical coupling the via the piezo-element. By measuring and evaluating electrical characteristic values of the piezo-element, it is possible to diagnose the mechanical function of the piezo-mechanical system; that is, to detect malfunctions and failures.
Besides the states of the mechanical subsystem, the state of the electrical subsystem (incl. piezo-element) can be diagnosed by the electrical characteristic values. Thus, for example, short circuits or leaks in the piezo-element lead to altered switching times of the system, owing to the altered impedance of the piezo-element, which times manifest themselves in various ways during the making and breaking phases in the time-dependent behavior of electrical characteristic values.
One diagnosis option is highly advantageous in the automotive field, for example, in which the legal regulation xe2x80x9cOBD2xe2x80x9d stipulates that all parts of an auto that are relevant to its safety and environmental impact must be monitored during the operation of a vehicle with respect to their proper functioning. This applies particularly to fuel injectors, the connection of which directly affects both the emission of pollutants and the reliable operation of a motor.
Another example is aeronautical construction, in which the mechanical movement of actuators, particularly of the elevators and vertical rudders, is determined by electrical signals (xe2x80x9cfly by wirexe2x80x9d). There are airplane constructions in existence which are not fit to fly without a permanent control and monitoring of the mechanical control parts. Malfunctions or damage at a mechanical drive, for instance at a linear drive with a piezoactuator, must therefore be detected early.